1. Field of the Invention
The present invention relates to a sinker electric discharge machining apparatus for forming in the workpiece a cavity which is complementary in shape to a tool electrode by applying a series of current pulses. In particular, the present invention relates to a machining condition determining system for determining machining conditions.
2. Description of the Related Art
In general, a tool electrode for sinker electric discharge machining is advanced along an axis of advancement to shape the workpiece. The axis of advancement is mostly a z-axis. In preparation for sinker electric discharge machining, a tool electrode is designed according to a three-dimensional geometry of the cavity being formed. Then, optimum machining conditions are determined in consideration of a three-dimensional geometry of the cavity, material of the tool electrode and workpiece and etc. The setting of the machining conditions are included in an NC program for electric discharge machining. The machining conditions are, for example, current peak, on-time and off-time of current pulse to be supplied to a tool electrode. Current peak is a critical machining condition which determines material removal rate and surface roughness. In general, a machining condition determining system considers machining area or active area of the tool electrode to determine current peak. The machining area is obtained by projecting the machining surface of the tool electrode downward on a plane perpendicular to the axis of advancement. If a column-shaped tool electrode is used, the machining area is an area of the base. As the machining progresses and the tool electrode advances further in the negative direction of z-axis, the machining area may be changed.
With the spread of CAD/CAM (Computer Aided Design/Computer Aided Manufacture), a CAD device now produces three-dimensional geometries of the cavity and tool electrode, and a CAM device automatically produces an NC program using such 3D geometry data. The 3D geometry data of the tool electrode or cavity is also used to accurately calculate the machining area.
Japanese Laid-open patent application 2003-291032 discloses a CAD device which produces a three-dimensional solid model of the tool electrode, and prepares a surface model of the tool electrode from the solid model. Machining area is calculated from the surface model for a plurality of depth positions of the tool electrode using an iterative method such as Newton-Raphson method. The optimum machining conditions are determined based on the machining area.
In order to obtain the optimum machining conditions, accurate calculation of the machining area is needed. Machining area calculation methods depend on a three-dimensional geometry of the tool electrode. Therefore, a conventional machining condition determining system must store a plurality of computation methods for circular cone, cylindrical column and etc.